1. Field of The Invention
The present invention relates to convertible sprockets for film and other media for accomodating different media formats, and more specifically to a convertible sprocket which can accommodate more than one format, such as more than one frame pitch for the medium.
2. Related Art
Conventional 35 mm film cameras and projectors operate with 35 mm film having a film frame pitch of four perforations per frame. The film is transported at a rate of 24 frames per second and 90 feet per minute. Film recording and projection equipment have long been standardized to operate with the four perforation frame pitch for 35 mm film. Therefore, equipment such as projectors use sprockets and pull-down assemblies specifically designed to operate with 35 mm film having four perforation frame pitch. The sprockets for conventional 35 mm film have 16 teeth around the circumferential surfaces. The frame pitch is 0.748 inches and the corresponding sprocket diameter for a 16 tooth sprocket is 0.952 inches.
Efforts have been made recently to more efficiently use available film stock by putting more film frames in a given unit length of film stock. One proposal has been to convert from a 4 perforation film frame pitch to a 3 perforation film frame pitch to eliminate 25 percent of unused film stock. Other film frame pitches and formats have also been proposed. To accommodate these different film formats and still project the film at a frame rate of 24 frames per second, according to the North American Standard, the projection and other film handling equipment must be modified to accommodate the changed format. It is believed that more than one frame pitch for motion picture equipment will be adopted by the film industry. While the standard Edison frame pitch of 4 perforations will remain useful, other intermittent pull-down pitches will be used to meet specific needs, such as presentation quality, economy, or efficiency. For example, possible additional film formats may require picture frames having perforation pitches of 5 perforations, 3 perforations, 21/2 or 2 perforations. However, changing from one film format to another for a given piece of equipment such as a projector is a time consuming and tedious process. Moreover, once a given piece of equipment is converted, the previous 4 perforation frame pitch can no longer be used with the equipment unless the sprocket assemblies and other components are reconverted. Conversion back and forth between the standard 4 perforation frame pitch and any modification will be necessary if more than one film format is to be accommodated in a projector or other piece of equipment. According to one approach, sprockets and other components of a projector or other piece of equipment for a given film format can be maintained on hand so that the projector can be converted at will. However, the conversion is a time consuming process, to be accomplished by film projectionists who would have to be trained to accomplish the conversion. Additionally, maintaining a complete set of conversion components may prove difficult because of possible loss or damage during the conversion process or while the equipment is being stored.
In an alternative approach, a sprocket conversion assembly may be used which has an outer cylindrical sleeve with teeth for engaging film perforations when the film is run according to one format and an inner cylindrical sleeve to fit within and anti-rotationally mate with the outer cylindrical sleeve. The inner sleeve has teeth for engaging film perforations according to an alternative format. Such an arrangement may permit less time consuming and costly conversions from one film format to another. However, separate parts are still required and the outer sleeve may be lost or damaged when not in use. Additionally, engagement of the tines on the inner sleeve with mating grooves on the inside of the outer sleeve may wear the tines of the inner sleeve, which may adversely effect film transport when using the inner sleeve.
There is a need, therefore, for a convertible sprocket which can be used to convert a given piece of equipment from one film format to one or more other film formats. Preferably, such a convertible sprocket is a single component which can accommodate the two or more film formats without having to store or remove additional equipment. the convertible sprocket is preferably easy to use and easily retrofit onto current equipment.